1. The Field of the Invention
The invention generally relates to the field of multiplexed fiber-optic communication. More specifically, the invention relates to methods and apparatus for extracting and replacing channels on a wavelength division multiplexed optical signal using an optical add/drop module.
2. Related Technology
Fiber-optics are increasingly used for transmitting voice and data signals. As a transmission medium, light provides a number of advantages over traditional electrical communication techniques. For example, light signals allow for extremely high transmission rates and very high bandwidth capabilities. Also, light signals are resistant to electromagnetic interference that would otherwise interfere with electrical signals. Light also provides a more secure signal because it does not emanate the type of high frequency components often experienced with conductor-based electrical signals. Many conventional electrical networks are being upgraded to optical networks to take advantage of the increased speed and efficiency.
One goal in optical fiber networks is to increase the amount of data traffic that can be transmitted through a single optical fiber. One way of increasing the amount of data traffic transmitted through an optical fiber is by using various types of multiplexing arrangements. One such multiplexing arrangement, referred to as Wavelength Division Multiplexing (WDM), is based on sending multiple signals down the same optical fiber where each signal is a modulated carrier beam. Each of the carrier beams has a different frequency or wavelength than the other carrier beams on a particular fiber. In Coarse Wavelength Division Multiplexing (CWDM), signals are sent using lasers with wavelengths varying between 1470 nm and 1610 nm in 20 nm increments. Each wavelength represents a different data channel. Another example of WDM is Dense Wavelength Division Multiplexing (WDM). In DWDM, channels are sent using lasers with wavelengths that are much closer together than in CWDM.
To effectively use the channels in a multiplexed signal, the channels need to be extracted from the multiplexed signal. This can be accomplished using a number of devices including optical add/drop modules and interleavers. Optical add/drop modules extract a single channel from the multiplexed signal and allow the single channel to be routed to a device having need of the channel. Optical add/drop modules also allow for a channel to be input to the optical add/drop module (typically the channel that was dropped), and combine the channel input into the optical add/drop module with the multiplexed signal.
Another component that can be used to extract channels from a multiplexed signal is an interleaver. Interleavers generally divide channels in a multiplexed signal into two different groups of channels. Thus, for a multiplexed signal with several channels, a number of interleavers may be necessary to extract a single channel. For example, a first stage interleaver divides a multiplexed signal of eight channels into two four-channel, multiplexed signals. A second stage interleaver divides the four channel, multiplexed signal into two two-channel, multiplexed signals. A third stage interleaver divides a two-channel, multiplexed signal into two single-channel signals.
Each channel is designed with a certain bandwidth to allow the carrier channel some tolerance in its actual frequency. One problem that arises in such multiplexing arrangements is that when a carrier channel becomes too near an adjacent channels bandwidth, the channel may cross into the adjacent channel and be interpreted as a signal in the adjacent channel. This is commonly referred to as cross talk.
Fused fiber interleavers have a Gaussian response. If the response of fused fiber interleavers is widened, then cross talk become more problematic. If the response of fused fiber interleavers is narrowed, then the fused fiber cannot handle variations in the wavelengths of the channels in a multiplexed optical signal.